Nowadays, ICs are rapidly becoming complex multi-functional devices by the inclusion of several types of functionality that extends beyond the traditional computational and signal processing tasks. For instance, ICs may include sensor functionality such that the IC can be used as a monitoring device in a wide range of technical fields, e.g. medical application domains, food processing and storage, mobile telecommunication, automotive and so on. Sensor designs may include temperature sensors, relative humidity sensors, gas sensors (e.g. O2, CO, CO2), analyte sensors (e.g. K+, Na+, glucose, pH) and pressure sensors, for instance.
Although many sensor designs are known per se, integration of such a sensor design on an integrated circuit is usually far from trivial because the required miniaturization, e.g. when moving from a large scale design to IC scale, poses many non-trivial problems, not in the least because the process steps used to realize a sensor design at a larger scale cannot be used in an IC manufacturing process, such that alternative ways of realizing such designs must be uncovered.
In addition, miniaturized sensors included in an IC design may suffer from problems that are the direct result of the miniaturization, such that it is not immediately apparent how such problems can be overcome, even if the solution is found in a design that may be known per se.
An example of a known pressure sensor integrated on an IC is shown in FIG. 1. The capacitive pressure sensor is formed on a semiconductor substrate 10 and comprises first electrode 20 on the substrate 10 and a diaphragm electrode 30 over the first electrode 20, thereby defining a cavity 25 in which the first electrode 20 is formed. Under the exertion of a pressure P on the diaphragm electrode 30, this electrode deforms towards the first electrode 20, thus causing a change in the capacitance C as the distance between the first electrode 20 and the diaphragm electrode 30 is altered. This type of pressure sensor is attractive because it can be easily integrated in an IC design. However, it does suffer from some noticeable drawbacks. The sensor is typically operated by placing an (alternating) electric potential across the electrodes 20 and 30. This potential exerts a force on the diaphragm electrode 30, which can deform under this force, thus reducing the accuracy of the sensor. Also, as one of the electrodes is typically charged, this charged electrode becomes sensitive to nearby electric fields (e.g. from other circuit components) such that additional shielding, e.g. in the form of a ground shield in the substrate 10, is necessary to protect the electrode from such adjacent electric fields, which adds to the complexity of the IC design.